Real Estate and Accumulated Depreciation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Real Estate Properties [Line Items]
Initial Basis	$ 984,390
Cost capitalized subsequent to acquisition	1,097,207
Land	453,322
Buildings	1,292,631
Construction in progress	335,644	$ 185,972
Total Purchase Price	2,081,597	1,948,165	$ 1,803,200	$ 1,700,813
Accumulated Depreciation	563,474	525,518	$ 488,166	$ 458,279
Book Value	1,518,123	1,422,647
Related Debt	821,503	880,271
Development land, related debt	108,623	$ 21,655
Total, related debt	938,421
Shopping Centers
Real Estate Properties [Line Items]
Initial Basis	841,335
Cost capitalized subsequent to acquisition	449,152
Land	415,424
Buildings	840,044
Construction in progress	35,019
Total Purchase Price	1,290,487
Accumulated Depreciation	381,382
Book Value	909,105
Related Debt	578,266
Shopping Centers | Ashbrook Marketplace, Ashburn, VA
Real Estate Properties [Line Items]
Initial Basis	8,938
Cost capitalized subsequent to acquisition	16,261
Land	3,066
Buildings	3,005
Construction in progress	19,128
Total Purchase Price	25,199
Accumulated Depreciation	8
Book Value	25,191
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Ashburn Village, Ashburn, VA
Real Estate Properties [Line Items]
Initial Basis	$ 11,431
Cost capitalized subsequent to acquisition	20,514
Land	6,764
Buildings	25,077
Construction in progress	104
Total Purchase Price	31,945
Accumulated Depreciation	14,234
Book Value	17,711
Related Debt	$ 26,245
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Ashland Square Phase I, Dumfries, VA
Real Estate Properties [Line Items]
Initial Basis	$ 1,178
Cost capitalized subsequent to acquisition	7,508
Land	1,178
Buildings	5,298
Construction in progress	2,210
Total Purchase Price	8,686
Accumulated Depreciation	2,215
Book Value	6,471
Related Debt	$ 0
Shopping Centers | Ashland Square Phase I, Dumfries, VA | Minimum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	20 years
Shopping Centers | Ashland Square Phase I, Dumfries, VA | Maximum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Beacon Center, Alexandria, VA
Real Estate Properties [Line Items]
Initial Basis	$ 24,161
Cost capitalized subsequent to acquisition	18,477
Land	22,674
Buildings	19,946
Construction in progress	18
Total Purchase Price	42,638
Accumulated Depreciation	15,335
Book Value	27,303
Related Debt	$ 36,206
Shopping Centers | Beacon Center, Alexandria, VA | Minimum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Beacon Center, Alexandria, VA | Maximum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | BJ’s Wholesale Club, Alexandria, VA
Real Estate Properties [Line Items]
Initial Basis	$ 22,623
Cost capitalized subsequent to acquisition	0
Land	22,623
Buildings	0
Construction in progress	0
Total Purchase Price	22,623
Accumulated Depreciation	0
Book Value	22,623
Related Debt	$ 10,323
Buildings and Improvements Depreciable Lives in Years	0 years
Shopping Centers | Boca Valley Plaza, Boca Raton, FL
Real Estate Properties [Line Items]
Initial Basis	$ 16,720
Cost capitalized subsequent to acquisition	2,064
Land	5,735
Buildings	13,049
Construction in progress	0
Total Purchase Price	18,784
Accumulated Depreciation	5,252
Book Value	13,532
Related Debt	$ 9,234
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Boulevard, Fairfax, VA
Real Estate Properties [Line Items]
Initial Basis	$ 4,883
Cost capitalized subsequent to acquisition	4,709
Land	3,687
Buildings	5,905
Construction in progress	0
Total Purchase Price	9,592
Accumulated Depreciation	3,243
Book Value	6,349
Related Debt	$ 15,723
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Briggs Chaney MarketPlace, Silver Spring, MD
Real Estate Properties [Line Items]
Initial Basis	$ 27,037
Cost capitalized subsequent to acquisition	4,703
Land	9,789
Buildings	21,951
Construction in progress	0
Total Purchase Price	31,740
Accumulated Depreciation	9,333
Book Value	22,407
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Broadlands Village, Ashburn, VA
Real Estate Properties [Line Items]
Initial Basis	$ 5,316
Cost capitalized subsequent to acquisition	34,586
Land	5,300
Buildings	34,589
Construction in progress	13
Total Purchase Price	39,902
Accumulated Depreciation	12,872
Book Value	27,030
Related Debt	$ 31,221
Shopping Centers | Broadlands Village, Ashburn, VA | Minimum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Broadlands Village, Ashburn, VA | Maximum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Burtonsville Town Square, Burtonsville, MD
Real Estate Properties [Line Items]
Initial Basis	$ 74,212
Cost capitalized subsequent to acquisition	5,612
Land	28,401
Buildings	50,915
Construction in progress	508
Total Purchase Price	79,824
Accumulated Depreciation	3,728
Book Value	76,096
Related Debt	$ 36,975
Shopping Centers | Burtonsville Town Square, Burtonsville, MD | Minimum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	20 years
Shopping Centers | Burtonsville Town Square, Burtonsville, MD | Maximum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	45 years
Shopping Centers | Countryside Marketplace, Sterling, VA
Real Estate Properties [Line Items]
Initial Basis	$ 28,912
Cost capitalized subsequent to acquisition	4,143
Land	7,666
Buildings	25,389
Construction in progress	0
Total Purchase Price	33,055
Accumulated Depreciation	10,518
Book Value	22,537
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Cranberry Square, Westminster, MD
Real Estate Properties [Line Items]
Initial Basis	$ 31,578
Cost capitalized subsequent to acquisition	739
Land	6,700
Buildings	25,617
Construction in progress	0
Total Purchase Price	32,317
Accumulated Depreciation	5,497
Book Value	26,820
Related Debt	$ 15,917
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Cruse MarketPlace, Cumming, GA
Real Estate Properties [Line Items]
Initial Basis	$ 12,226
Cost capitalized subsequent to acquisition	689
Land	3,901
Buildings	9,014
Construction in progress	0
Total Purchase Price	12,915
Accumulated Depreciation	3,626
Book Value	9,289
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Flagship Center, Rockville, MD
Real Estate Properties [Line Items]
Initial Basis	$ 160
Cost capitalized subsequent to acquisition	9
Land	169
Buildings	0
Construction in progress	0
Total Purchase Price	169
Accumulated Depreciation	0
Book Value	169
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	0 years
Shopping Centers | French Market, Oklahoma City, OK
Real Estate Properties [Line Items]
Initial Basis	$ 5,781
Cost capitalized subsequent to acquisition	14,269
Land	1,118
Buildings	18,929
Construction in progress	3
Total Purchase Price	20,050
Accumulated Depreciation	12,490
Book Value	7,560
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Germantown, Germantown, MD
Real Estate Properties [Line Items]
Initial Basis	$ 2,034
Cost capitalized subsequent to acquisition	567
Land	2,034
Buildings	567
Construction in progress	0
Total Purchase Price	2,601
Accumulated Depreciation	409
Book Value	2,192
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | The Glen, Woodbridge, VA
Real Estate Properties [Line Items]
Initial Basis	$ 12,918
Cost capitalized subsequent to acquisition	8,351
Land	5,300
Buildings	15,942
Construction in progress	27
Total Purchase Price	21,269
Accumulated Depreciation	10,138
Book Value	11,131
Related Debt	$ 22,448
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Great Falls Center, Great Falls, VA
Real Estate Properties [Line Items]
Initial Basis	$ 41,750
Cost capitalized subsequent to acquisition	3,203
Land	14,766
Buildings	30,187
Construction in progress	0
Total Purchase Price	44,953
Accumulated Depreciation	9,497
Book Value	35,456
Related Debt	$ 10,774
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Hampshire Langley, Takoma, MD
Real Estate Properties [Line Items]
Initial Basis	$ 3,159
Cost capitalized subsequent to acquisition	3,549
Land	1,856
Buildings	4,635
Construction in progress	217
Total Purchase Price	6,708
Accumulated Depreciation	3,908
Book Value	2,800
Related Debt	$ 14,810
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Hunt Club Corners, Apopka, FL
Real Estate Properties [Line Items]
Initial Basis	$ 12,584
Cost capitalized subsequent to acquisition	4,409
Land	4,822
Buildings	12,171
Construction in progress	0
Total Purchase Price	16,993
Accumulated Depreciation	4,788
Book Value	12,205
Related Debt	$ 5,300
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Jamestown Place, Altamonte Springs, FL
Real Estate Properties [Line Items]
Initial Basis	$ 14,055
Cost capitalized subsequent to acquisition	1,950
Land	4,455
Buildings	11,550
Construction in progress	0
Total Purchase Price	16,005
Accumulated Depreciation	4,291
Book Value	11,714
Related Debt	$ 6,539
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Kentlands Square I, Gaithersburg, MD
Real Estate Properties [Line Items]
Initial Basis	$ 14,379
Cost capitalized subsequent to acquisition	840
Land	5,006
Buildings	9,711
Construction in progress	502
Total Purchase Price	15,219
Accumulated Depreciation	4,220
Book Value	10,999
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Kentlands Square II, Gaithersburg, MD
Real Estate Properties [Line Items]
Initial Basis	$ 76,723
Cost capitalized subsequent to acquisition	2,733
Land	22,800
Buildings	56,270
Construction in progress	386
Total Purchase Price	79,456
Accumulated Depreciation	12,147
Book Value	67,309
Related Debt	$ 33,952
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Kentlands Place, Gaithersburg, MD
Real Estate Properties [Line Items]
Initial Basis	$ 1,425
Cost capitalized subsequent to acquisition	7,373
Land	1,425
Buildings	7,373
Construction in progress	0
Total Purchase Price	8,798
Accumulated Depreciation	4,241
Book Value	4,557
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Lansdowne Town Center, Leesburg, VA
Real Estate Properties [Line Items]
Initial Basis	$ 6,545
Cost capitalized subsequent to acquisition	43,003
Land	6,546
Buildings	37,843
Construction in progress	5,159
Total Purchase Price	49,548
Accumulated Depreciation	16,291
Book Value	33,257
Related Debt	$ 30,719
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Leesburg Pike Plaza, Baileys Crossroads, VA
Real Estate Properties [Line Items]
Initial Basis	$ 2,418
Cost capitalized subsequent to acquisition	6,292
Land	1,132
Buildings	7,578
Construction in progress	0
Total Purchase Price	8,710
Accumulated Depreciation	6,159
Book Value	2,551
Related Debt	$ 14,414
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Lumberton Plaza, Lumberton, NJ
Real Estate Properties [Line Items]
Initial Basis	$ 4,400
Cost capitalized subsequent to acquisition	11,612
Land	950
Buildings	15,041
Construction in progress	21
Total Purchase Price	16,012
Accumulated Depreciation	13,379
Book Value	2,633
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Metro Pike Center, Rockville, MD
Real Estate Properties [Line Items]
Initial Basis	$ 33,123
Cost capitalized subsequent to acquisition	4,605
Land	26,064
Buildings	7,729
Construction in progress	3,935
Total Purchase Price	37,728
Accumulated Depreciation	1,822
Book Value	35,906
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Shops at Monocacy, Frederick, MD
Real Estate Properties [Line Items]
Initial Basis	$ 9,541
Cost capitalized subsequent to acquisition	13,994
Land	9,260
Buildings	14,275
Construction in progress	0
Total Purchase Price	23,535
Accumulated Depreciation	6,338
Book Value	17,197
Related Debt	$ 28,500
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Northrock, Warrenton, VA
Real Estate Properties [Line Items]
Initial Basis	$ 12,686
Cost capitalized subsequent to acquisition	15,429
Land	12,686
Buildings	15,423
Construction in progress	6
Total Purchase Price	28,115
Accumulated Depreciation	4,907
Book Value	23,208
Related Debt	$ 14,085
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Olde Forte Village, Ft. Washington, MD
Real Estate Properties [Line Items]
Initial Basis	$ 15,933
Cost capitalized subsequent to acquisition	6,721
Land	5,409
Buildings	17,245
Construction in progress	0
Total Purchase Price	22,654
Accumulated Depreciation	8,180
Book Value	14,474
Related Debt	$ 21,702
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Olney, Olney, MD
Real Estate Properties [Line Items]
Initial Basis	$ 4,963
Cost capitalized subsequent to acquisition	2,550
Land	3,079
Buildings	4,434
Construction in progress	0
Total Purchase Price	7,513
Accumulated Depreciation	3,358
Book Value	4,155
Related Debt	$ 11,952
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Orchard Park, Dunwoody, GA
Real Estate Properties [Line Items]
Initial Basis	$ 19,377
Cost capitalized subsequent to acquisition	1,461
Land	7,751
Buildings	13,087
Construction in progress	0
Total Purchase Price	20,838
Accumulated Depreciation	4,053
Book Value	16,785
Related Debt	$ 9,441
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Palm Springs Center, Altamonte Springs, FL
Real Estate Properties [Line Items]
Initial Basis	$ 18,365
Cost capitalized subsequent to acquisition	2,007
Land	5,739
Buildings	14,633
Construction in progress	0
Total Purchase Price	20,372
Accumulated Depreciation	5,593
Book Value	14,779
Related Debt	$ 7,262
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Ravenwood, Baltimore, MD
Real Estate Properties [Line Items]
Initial Basis	$ 1,245
Cost capitalized subsequent to acquisition	4,256
Land	703
Buildings	4,798
Construction in progress	0
Total Purchase Price	5,501
Accumulated Depreciation	3,308
Book Value	2,193
Related Debt	$ 13,606
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | 11503 Rockville Pike/5541 Nicholson Lane, Rockville, MD
Real Estate Properties [Line Items]
Initial Basis	$ 26,561
Cost capitalized subsequent to acquisition	24
Land	22,113
Buildings	4,472
Construction in progress	0
Total Purchase Price	26,585
Accumulated Depreciation	1,032
Book Value	25,553
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | 1500/1580/1582/1584 Rockville Pike, Rockville, MD
Real Estate Properties [Line Items]
Initial Basis	$ 51,149
Cost capitalized subsequent to acquisition	2,714
Land	43,863
Buildings	7,490
Construction in progress	2,510
Total Purchase Price	53,863
Accumulated Depreciation	6,439
Book Value	47,424
Related Debt	0
Shopping Centers | Seabreeze Plaza, Palm Harbor, FL
Real Estate Properties [Line Items]
Initial Basis	24,526
Cost capitalized subsequent to acquisition	2,502
Land	8,665
Buildings	18,363
Construction in progress	0
Total Purchase Price	27,028
Accumulated Depreciation	6,746
Book Value	20,282
Related Debt	$ 15,019
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Market Place at Sea Colony, Bethany Beach, DE
Real Estate Properties [Line Items]
Initial Basis	$ 2,920
Cost capitalized subsequent to acquisition	246
Land	1,147
Buildings	2,019
Construction in progress	0
Total Purchase Price	3,166
Accumulated Depreciation	615
Book Value	2,551
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Seven Corners, Falls Church, VA
Real Estate Properties [Line Items]
Initial Basis	$ 4,848
Cost capitalized subsequent to acquisition	44,612
Land	4,913
Buildings	44,522
Construction in progress	25
Total Purchase Price	49,460
Accumulated Depreciation	30,937
Book Value	18,523
Related Debt	$ 60,677
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Severna Park Marketplace, Severna Park, MD
Real Estate Properties [Line Items]
Initial Basis	$ 63,254
Cost capitalized subsequent to acquisition	526
Land	12,700
Buildings	51,080
Construction in progress	0
Total Purchase Price	63,780
Accumulated Depreciation	10,544
Book Value	53,236
Related Debt	$ 29,710
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Shops at Fairfax, Fairfax, VA
Real Estate Properties [Line Items]
Initial Basis	$ 2,708
Cost capitalized subsequent to acquisition	9,971
Land	992
Buildings	11,687
Construction in progress	0
Total Purchase Price	12,679
Accumulated Depreciation	8,676
Book Value	4,003
Related Debt	$ 10,482
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | Smallwood Village Center, Waldorf, MD
Real Estate Properties [Line Items]
Initial Basis	$ 17,819
Cost capitalized subsequent to acquisition	8,332
Land	6,402
Buildings	19,743
Construction in progress	6
Total Purchase Price	26,151
Accumulated Depreciation	8,903
Book Value	17,248
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Southdale, Glen Burnie, MD
Real Estate Properties [Line Items]
Initial Basis	$ 18,895
Cost capitalized subsequent to acquisition	25,100
Land	15,254
Buildings	28,740
Construction in progress	1
Total Purchase Price	43,995
Accumulated Depreciation	22,440
Book Value	21,555
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Southside Plaza, Richmond, VA
Real Estate Properties [Line Items]
Initial Basis	$ 6,728
Cost capitalized subsequent to acquisition	11,338
Land	1,878
Buildings	16,188
Construction in progress	0
Total Purchase Price	18,066
Accumulated Depreciation	12,798
Book Value	5,268
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | South Dekalb Plaza, Atlanta, GA
Real Estate Properties [Line Items]
Initial Basis	$ 2,474
Cost capitalized subsequent to acquisition	4,449
Land	703
Buildings	6,220
Construction in progress	0
Total Purchase Price	6,923
Accumulated Depreciation	5,088
Book Value	1,835
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Thruway, Winston-Salem, NC
Real Estate Properties [Line Items]
Initial Basis	$ 7,848
Cost capitalized subsequent to acquisition	26,336
Land	7,693
Buildings	26,447
Construction in progress	44
Total Purchase Price	34,184
Accumulated Depreciation	18,682
Book Value	15,502
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Village Center, Centreville, VA
Real Estate Properties [Line Items]
Initial Basis	$ 16,502
Cost capitalized subsequent to acquisition	2,636
Land	7,851
Buildings	11,287
Construction in progress	0
Total Purchase Price	19,138
Accumulated Depreciation	7,380
Book Value	11,758
Related Debt	$ 12,555
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Westview Village, Frederick, MD
Real Estate Properties [Line Items]
Initial Basis	$ 6,047
Cost capitalized subsequent to acquisition	25,225
Land	6,047
Buildings	25,216
Construction in progress	9
Total Purchase Price	31,272
Accumulated Depreciation	9,483
Book Value	21,789
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	50 years
Shopping Centers | White Oak, Silver Spring, MD
Real Estate Properties [Line Items]
Initial Basis	$ 6,277
Cost capitalized subsequent to acquisition	5,771
Land	4,649
Buildings	7,212
Construction in progress	187
Total Purchase Price	12,048
Accumulated Depreciation	6,169
Book Value	5,879
Related Debt	$ 22,475
Buildings and Improvements Depreciable Lives in Years	40 years
Shopping Centers | Other Buildings / Improvements
Real Estate Properties [Line Items]
Cost capitalized subsequent to acquisition	$ 182
Buildings	182
Construction in progress	0
Total Purchase Price	182
Accumulated Depreciation	72
Book Value	110
Related Debt	0
Mixed-Use Properties
Real Estate Properties [Line Items]
Initial Basis	43,967
Cost capitalized subsequent to acquisition	437,116
Land	28,496
Buildings	452,587
Construction in progress	0
Total Purchase Price	481,083
Accumulated Depreciation	182,092
Book Value	298,991
Related Debt	$ 249,956
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date One
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	5 years
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date Two
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	10 years
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date Three
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	5 years
Mixed-Use Properties | 1500/1580/1582/1584 Rockville Pike, Rockville, MD | Acquisition Date Four
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	4 years
Mixed-Use Properties | Avenel Business Park, Gaithersburg, MD
Real Estate Properties [Line Items]
Initial Basis	$ 21,459
Cost capitalized subsequent to acquisition	32,976
Land	3,756
Buildings	50,679
Construction in progress	0
Total Purchase Price	54,435
Accumulated Depreciation	39,261
Book Value	15,174
Related Debt	$ 26,260
Mixed-Use Properties | Avenel Business Park, Gaithersburg, MD | Minimum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	35 years
Mixed-Use Properties | Avenel Business Park, Gaithersburg, MD | Maximum
Real Estate Properties [Line Items]
Buildings and Improvements Depreciable Lives in Years	40 years
Mixed-Use Properties | Clarendon Center, Arlington, VA (1)
Real Estate Properties [Line Items]
Initial Basis	$ 12,753
Cost capitalized subsequent to acquisition	186,210
Land	16,287
Buildings	182,676
Construction in progress	0
Total Purchase Price	198,963
Accumulated Depreciation	46,856
Book Value	152,107
Related Debt	$ 98,611
Buildings and Improvements Depreciable Lives in Years	50 years
Mixed-Use Properties | Park Van Ness, Washington, DC
Real Estate Properties [Line Items]
Initial Basis	$ 2,242
Cost capitalized subsequent to acquisition	91,699
Land	2,242
Buildings	91,699
Construction in progress	0
Total Purchase Price	93,941
Accumulated Depreciation	10,757
Book Value	83,184
Development land, related debt	$ 68,095
Buildings and Improvements Depreciable Lives in Years	50 years
Mixed-Use Properties | 601 Pennsylvania Ave., Washington, DC
Real Estate Properties [Line Items]
Initial Basis	$ 5,479
Cost capitalized subsequent to acquisition	69,129
Land	5,667
Buildings	68,941
Construction in progress	0
Total Purchase Price	74,608
Accumulated Depreciation	57,038
Book Value	17,570
Related Debt	$ 0
Buildings and Improvements Depreciable Lives in Years	35 years
Mixed-Use Properties | Washington Square, Alexandria, VA
Real Estate Properties [Line Items]
Initial Basis	$ 2,034
Cost capitalized subsequent to acquisition	57,102
Land	544
Buildings	58,592
Construction in progress	0
Total Purchase Price	59,136
Accumulated Depreciation	28,180
Book Value	30,956
Related Debt	$ 56,990
Buildings and Improvements Depreciable Lives in Years	50 years
Development Land
Real Estate Properties [Line Items]
Initial Basis	$ 99,088
Cost capitalized subsequent to acquisition	210,939
Land	9,402
Buildings	0
Construction in progress	300,625
Total Purchase Price	310,027
Accumulated Depreciation	0
Book Value	310,027
Development land, related debt	110,199
Development Land | Ashland Square Phase II, Manassas, VA
Real Estate Properties [Line Items]
Initial Basis	5,292
Cost capitalized subsequent to acquisition	2,280
Land	7,028
Buildings	0
Construction in progress	544
Total Purchase Price	7,572
Accumulated Depreciation	0
Book Value	7,572
Development land, related debt	0
Development Land | New Market, New Market, MD
Real Estate Properties [Line Items]
Initial Basis	2,088
Cost capitalized subsequent to acquisition	286
Land	2,374
Buildings	0
Construction in progress	0
Total Purchase Price	2,374
Accumulated Depreciation	0
Book Value	2,374
Development land, related debt	0
Development Land | The Waycroft, Arlington, VA
Real Estate Properties [Line Items]
Initial Basis	52,067
Cost capitalized subsequent to acquisition	203,376
Land	0
Buildings	0
Construction in progress	255,443
Total Purchase Price	255,443
Accumulated Depreciation	0
Book Value	255,443
Development land, related debt	110,199
Development Land | 7316 Wisconsin Avenue, Bethesda, MD
Real Estate Properties [Line Items]
Initial Basis	39,641
Cost capitalized subsequent to acquisition	4,997
Land	0
Buildings	0
Construction in progress	44,638
Total Purchase Price	44,638
Accumulated Depreciation	0
Book Value	44,638
Development land, related debt	$ 0
Buildings and Improvements Depreciable Lives in Years